Often, it is necessary in an industrial or other process to inject a measured quantity of a flowable material into a further stream of material or a vessel. Metering pumps have been developed for this purpose and may be either electromagnetically or hydraulically actuated. Conventionally, an electromagnetic metering pump utilizes a linear solenoid which is provided half-wave or full-wave rectified pulses to move a diaphragm mechanically linked to an armature of the solenoid.
FIGS. 1 and 2 illustrate a conventional control strategy for an electromagnetic metering pump 15 (shown in FIG. 3). A solenoid 16 (also shown in FIG. 3) is electrically powered at a sufficient level to provide a pumping force at maximum air gap (i.e., zero stroke) which will meet or exceed the maximum fluid force expected to be encountered. The electric power is also delivered at maximum power level at all other stroke positions.
As illustrated in FIG. 3, the stroke length of the metering pump 15 is conventionally controlled by a mechanical stroke length adjustment control 17 comprising a screw 18 and a handle 19. Typically, an operator of the pump manually sets the stroke length by turning the handle 19, thereby adjusting the screw 18 to a position corresponding to the desired stroke length.
Moreover, the metering pump is ordinarily primed by operating a priming button disposed external to the pump. To prime in this manner, the operator first manually adjusts the mechanical stroke length adjustment control 17 via the handle 19 to the position associated with a maximum stroke length and then pushes the external prime button, which in turn causes the pump to run at its maximum pumping rate.
Several problems, however, arise during the operation of the conventional metering pump. First, the heat that is generated by the electrical powering of the solenoid typically results in the need for components that can tolerate same, such as plastic and metal enclosures and other plastic and metal parts and/or larger solenoids with more copper windings. In addition, the extra forces applied to the armature in light of the maximum power that is applied result in the need for relatively heavier return springs and components to counteract residual magnetism and allow the armature to return in time for the pump diaphragm to do suction work. Still further, sound levels are increased owing to the banging of the armature at the end of the stroke when pumping against lower force levels, and further due to the striking of the armature against a stroke adjustment stop at the end of each suction stroke under the influence of the heavy return spring. Service life is typically short owing to the mechanical stresses that are encountered.
In addition, the conventional mechanical stroke length adjustment control 17 can be inaccurate owing to a lack of precision of the parts and wear.
Moreover, the priming devices present in even the most sophisticated metering pumps are not capable of automatically detecting a loss of prime. Rather, the operator must independently detect that a loss-of-prime condition has arisen. In addition, conventional metering pumps do not automatically return to the originally programmed stroke settings or pump operating conditions after priming or repriming.